Evolution: lizards > mice > monkeys > humans

If you’ve subscribed to this theory to how our brains have evolved, I’m sorry (not really) to tell you cancel that subscription and get ready to be schooled on why the reptilian brain theory is a myth. Most of you nerds may be familiar with the widely-used introductory textbook in psychology (Myers & Dewall, 2018, p. 68) which states:

“In primitive animals, such as sharks, a not-so-complex brain primarily regulates basic survival functions… In lower mammals, such as rodents, a more complex brain enables emotion and greater memory… In advanced mammals, such as humans, a brain that processes more information enables increased foresight as well… The brain’s increasing complexity arises from new brain systems built on top of the old, much as the Earth’s landscape covers the old with the new. Digging down, one discovers the fossil remnants of the past…”

The MacLean’s model suggests the human brain is organized into a hierarchy, which itself is based on an evolutionary view of brain development. The three regions are as follows:

  1. Reptilian or Primal Brain (Basal Ganglia)
  2. Paleomammalian or Emotional Brain (Limbic System)
  3. Neomammalian or Rational Brain (Neocortex)

Surprisingly, there are numerous papers that cite MacLean’s model of brain evolution in academic and non-academic articles in various sub-areas of psychology including models of personality (Epstein, 1994), attention (Mirsky & Duncan, 2002), psychopathology (Cory & Gardener, 2002), market economics (Cory, 2002), and morality (Narvaez, 2008)

Perhaps, there is an allure in a neat division of primal, emotional and rational mental activities between the 3 systems and it oversimplifies the human experience. Additionally, this model was a neuroanatomical cousin to Freud’s theory of personality: id, ego & superego, that neatly overlaps onto the reptilian, limbic and cortical brains respectively.

What’s the problem then?

  1. There is a lack in consistency of anatomical location of the reptilian brain. While McLean originally tagged it as the basal ganglia in humans, there have been multiple adaptations where it’s taken out of context to showcase ‘fight or flight response’ by labelling it as

2. Evolutionary biology disproves this linear model: lizards > mice > monkeys > humans. There wasn’t a linear progression over time, and we most definitely didn’t have lizard ancestors. Instead, mammals have radiated from common fish-like ancestors and within each sub-branch have independently evolved to form their own complex nervous systems since.

3. Addition of complex neural structures does not equate to increased behavioural complexity. The triune model neglects to look at the functional complexity of the added neuro structures. We can see this to be untrue in the case of non-human mammals that react flexibly to stimulus, and generate similar neurotransmitters and hormones to integrate information in the nervous system

Neuroscience deconstructs the ‘classical view’ of emotions which believes that the human mind is structured as a set of mental abilities, each associated with a unique state caused by its own, unique biological system

What is Emotion constructed by?

“How can a three-pound mass of jelly that you can hold in your palm imagine angels, contemplate the meaning of infinity, and even question its own place in the cosmos? Especially awe inspiring is the fact that any single brain, including yours, is made up of atoms that were forged in the hearts of countless, far-flung stars billions of years ago. These particles drifted for eons and light-years until gravity and change brought them together here, now. These atoms now form a conglomerate- your brain- that can not only ponder the very stars that gave it birth but can also think about its own ability

to think and wonder about its own ability to wonder. With the arrival of humans, it has been said, the universe has suddenly become conscious of itself. This, truly, it the greatest mystery of all.”

V.S. Ramachandran, The Tell-Tale Brain: A Neuroscientist’s Quest for What Makes Us Human

Emotions are constantly being accounted in our subconscious, even if we aren’t actively exhibiting a particular emotion. Our brain tends to act like a sponge and absorb patterns from loved ones, like how an infant mirrors his/her mother’s emotional state, and as we grow up and gain more exposure to the external environment, we learn to sort out the various signals. Our nervous circuits are modified at every junction when a new memory is made, new language is picked up, when exposed to a new culture. And so, we cannot attribute our ‘flight and fight response” to the sympathetic nervous system alone. The SNS releases noradrenaline from nerve endings and directly into the bloodstream increases heart rate and breathing and directs blood to the large leg muscles to allow us to run from danger quicker, just as it does for any prey or predator (lizards included). However, we need to recall the nuance that each of these species have a completely different set of emotions and motivations to activate the SNS. For the prey, it’s to escape the predator. For the predator, to catch the prey. For us humans, to escape from the attending who always asks endless questions about mitral valve prolapse. For all the above 3, the SNS has evolved to provide for the body’s energy demands, to generate more ATP to ensure that it can meet the body’s requirements in any situation- be it a threat or an opportunity.

The context makes the world of a difference as we come to a consensus that our lives are no longer controlled by primitive automated instincts that are mindless responses to a particular trigger.

Human First

Vyshnavi Desiraju

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One response

  1. Thanks for this post! I was literally taught the triune model less than two years ago, super insightful to know this has been debunked already! I think as much as this model isn’t relevant to current times, it did help lay the bricks for our current understanding of the evolving brain.

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